I never found much to be gained in comparing ruthlessness or the brutality of one dictator in relation to other dictators. Because comparing one leader/country's crimes to another leads to a mitigation effect of those who didn't commit violence on the scale of the Holocaust/Holodomor, and lead to an unhelpful or even dangerous "It wasn't that bad!" narrative. With that out of the way, lets talk about Big Chin.

With the founding and rise of the Fascist Party in Italy in the aftermath of WWI, the Blackshirts, along with its predecessor called the Squadrismo, began their efforts as little more than organized thugs who would intimidate opposition parties, their voters, and their leaders. Their tactics included physical beatings of those in opposition, kidnappings, and the murder of socialist party members. Street fights between Socialist Party supporters and the Blackshirts would erupt during elections in order to discourage voter turnout for the Socialists. Blackshirts would be stationed outside of polling places with clubs and pistols in order to further intimidate opposition voters into turning away. The street violence and kidnappings performed by the Blackshirts hit a boiling point with the kidnapping and murder of Giacomo Matteotti, a member of the chamber of deputies and leader of the Unitary Socialist Party in May of 1924, who was found stabbed and dumped out of a car few miles north of Rome.

Public outrage over the murder and accusations against Mussolini for organizing a cover-up of the murder were brought forward, but his direct involvement in the murder itself still had no solid proof. Opposition parties however failed to react and capitalize on the opportunity against Mussolini, in hopes that King Victor Emmanuel III would simply dismiss him over the allegations. Mussolini instead gave a speech on January 3, 1925 taking responsibility for the violence of the Black Shirts and essentially just dared anyone to stop him. No one challenged him and soon Italy's democratic institutions were stripped as Mussolini became and effective dictator and established a police state.

While the Italian fascists were responsible for street violence, kidnappings, and political murders on the Italian homeland, Italy's larger crimes were committed in the country's colonial possessions during the pacification of Libya, the 2nd Italo-Ethiopian War and WWII.

Starting in 1923, Mussolini's Italy carried out an extensive military operation to quell rebellion and unrest in the newly established colony of Libya, fighting a guerrilla war against local Libyan tribes and the Senussi Order, the largest and most organized of the Libyan resistance movements against Italian occupation. Italian soldiers, and later blackshirt divisions that have been incorporated into the military proper, carried out chemical weapon attacks, forced expulsions of entire populations, and mass executions of surrendered opposition soldiers in order to open up settlement space for Italian nationals in Libya. By the end of the pacification campaign in 1932, over 100,000 Cyrenaicans living in Eastern Libya were estimated to have been killed.

In 1935, Italy resumed its expansionist efforts in Africa by invading Ethiopia with an army of over half a million soldiers. The invasion and resulting occupation of Ethiopia saw the widespread use of mustard gas used against Ethiopian units by artillery, aerial bombing, and airplanes strafing and spraying the gas on Ethiopian positions as they made their diving runs. Italian units bombed and attacked Red Cross ambulances and staff to deny medical care to Ethiopian soldiers as well as mass executions of captured soldiers both during the conflict and during the occupation and pacification of the country after Ethiopia's surrender. During the occupation, Ethiopians attempted to assassinate the Italian viceroy of the newly established Italian East Africa colony, Field Marshal Rodolfo Graziani. Graziani's response to the attempt on his life led to what Ethiopians refer to as Yekatit 12, a series of indiscriminate mass executions and imprisonments of Ethiopians and Eritreans throughout 1937, where its estimated that up to 30,000 civilians were killed as part of Graziani's terror campaign.

During WWII, Italian occupation forces also took part in various war crimes in both occupied Yugoslavia and Greece. Italian authorities established concentration camps to hold political prisoners which could hold up to 50,000 prisoners, of which over 25,000 in present-day Slovenia died of forced starvation. In Greece, a similar program of forced starvation led to a famine that killed over 300,000 Greek civilians, as well as massacres of Greek civilians as reprisals for resistance activity, the most notable of which being the Domenikon massacre, with 175 Greek civilians executed in response to resistance activity in February of 1943.

Although these crimes have been overshadowed by the sheer scale of other crimes committed by Nazi Germany and the Soviet Union, the crimes committed by the Fascists of Italy are still there and should not be understated. Doing so would be a disservice to the hundreds of thousands of Libyans, Ethiopians, Slovenes, and Greeks who suffered and died under Fascism.

Sources:

The Rise of Italian Fascism: 1918-1922 by Angelo Rossi, Routledge. 2013.

Re-conquest and Suppression: Fascist Italy's Pacification of Libya and Ethiopia by John Gooch, Journal of Strategic Studies. 2005.

Fascism's European Empire: Italian Occupation During the Second World War by Davide Rodogno, Cambridge University Press. 2006.

I am not sure I know how to answer the question (thankfully /u/quiaudetvincet has already covered a lot of ground here).

There is no doubt that Fascism made use of violence in its affirmation – or, from a wider angle, that violence was a significant portion of the process which brought Fascism to assume the functions of main social and political power in Italy. Indeed the fascist movement was, since its beginnings, rather unapologetic about its violence, which was consistently framed as a natural and organic reaction of the “healthier” portions of the Nation against the equally violent, but far more unnatural and destructive, Bolshevik threat. This framing – of a “reaction” (but not reactionary) violence – was maintained throughout the process of development of Fascism and, I'd say, well into the normalization-institutionalization stage which saw its final crisis with the assassination of socialist representative Giacomo Matteotti in June 1924.

But here we are already running into a first major issue: fascist violence wasn't merely a consequence of Mussolini's ruthlessness, and whether he was ruthless or not, and how much, tells us very little of how and by what means Fascism came into existence and developed into its progressive stages of derivation. Violence in the early 1920s – and even before, during the war, and before the war as well, and also later on in the various administrative-police measures available to the Regime – let's say more or less always, when we look at violence in a social context, we can't escape the need to address violence itself as a social phenomenon, and therefore to take into account that it has a substantial structural portion.

Individual violence – in its purely individual form – can probably be abstracted from general social considerations, at least for the purpose of a general assessment. I would not feel in any way at odds with myself for claiming that Robert Hansen was, well, ruthless. But, already limited as this may appear for the purpose of examining individual behavior, I don't think it tells us anything useful in so far as social and collective behaviors go.

Now, if a were to speak from my general impression (which in good measure echoes that of De Felice), I'd say that Mussolini wasn't especially ruthless, in so far as his personal disposition went. He often displayed a natural fondness for acts of modest liberality, as long as his former enemies were open enough about their contrition and no longer able to inconvenience him. I wouldn't say that he was forgiving with those he deemed his equals though – and certainly he would think less of you if you didn't throw a punch when you had a good chance and a half-decent reason. This attitude certainly carried over to his quite narrow, particular conceit of realpolitik and to his inclination of mistaking power for crassness, where display and intellectual formation operated in substantial contiguity. In this sense, he didn't believe politics to be a place for ideological or moral reservations. I have written much more extensively about Mussolini's mindset here.

That said, there is (what I think is) a good example to illustrate why such approach can't lead us very far.

On March 22^nd 1922 a wife and husband from the province of Ferrara wrote the following letter to the Ministry of Interior. The man, a former leader of a local socialist league, with a “reputation” of his own, the woman, a former school teacher; they had both relocated to S. Marino and intended to explain their circumstances.

Having appealed without any fortune to the prefect Bladier [Gennaro, in Ferrara], to the King's Attorney in Ferrara, to the prefect Mori [Cesare, in Bologna], we wish to denounce [that] on April 5^th 1921 we were forced out of Baura, a town nearby Ferrara, where we used to have our residence: [the husband], born there, was head of the local cooperative, [the wife] was a teacher in the local school. There the authorities, under pretense of ignoring the fascist proscription, loudly declared out in a public road and in presence of the King's Carabineers, that “had we not left within the sixty hours, our house would have been burned to the ground and our bodies carved into pieces”, [and also] fired [the wife] after 16 years of teaching.

This decision was confirmed by the local fascists: Ghetti Waldem […] Tumiati Gino […] Felloni Dante and Felloni Antonio […] The most hostile of them being Tumiati Gino who, with his many brothers, had frequently stated in public his intention to murder [the husband].

You are probably thinking that it's a bit odd that the advice given by the unnamed local authorities, to relocate in order to avoid incidents, was framed in such a way that the two regarded the sanctioning given by the members of the local Fascio as the most significant piece of information for the Ministry of Interior.

But, as you can see, there is no act of violence here. No murder, or stabbing, or beating. No arson. Their house remained there. One could even doubt the exactness of their version. After all, it's just their word. And the husband was a notorious socialist, who had not shied away from conflicts during the years before the affirmation of the local Fascio.

What we have is the fact that they did leave, both their home and occupation. And came back in 1946. Which would suggest that the threat of violence was substantial enough not to require an execution.

Back in 1921, the prefect of Ferrara was Samuele Pugliese, who was notoriously sympathetic to the action of the Fascio. As he explained to the Ministry on March 12^th 1921

The Fascio di Combattimento in Ferrara has had, since its foundation, one political purpose: fight against any devaluation of the war and of the victory. In its development, the original manifesto changed content and focus, moving from the original conservation of a moral set of values, to offensive action, in order to free the workers' mass from [anyone who wished to place them under] ideological servitude. Therefore their aggressive method of fight; armed strikes in the neighboring localities, where requested [by the locals] or where the red organizations were alleged to have a stronger grip.

To better shine a light on those “aggressive methods”, we can continue to follow Pugliese (May 8^th 1921)

Last night 20 persons, having surrounded the house of Gaiba Natale […] took and brought on the street Gaiba himself, a notorious subversive, who, having attempted to escape, was hit by a gunshot, and left dead. Public Security […] identified and apprehended the murderer as Morandi Umberto and nine more and took custody of their guns.

This was, of course, an extreme outcome of a practice of violence that otherwise didn't require an overt act of violence, favoring instead, quite often, almost symbolic acts of violence – from the capture of the red flag, removed from public buildings and replaced with the national Tricolore, to local “subversives” being forces to sing war songs or to salute the flag – for the purpose of restoring the community of the nation within its natural, organic boundaries, removing only those who appeared irreducible elements of disturbance.

One last thing that should be addressed is how to reconcile the individual action, the intentional violence carried off in its material, practical terms, with the structural violence which is always present in society but which certainly assumed a special character in Fascist society. It's more of a philosophical question than a historical one.

We know fairly well that Mussolini had limited knowledge of the specific initiatives of the single Fasci, with actions such as those described above being usually arranged with the approval of local leaders. We also know that – despite a certain distance that he was keen on maintaining, and despite his (successful) efforts to credit himself as the one man who could institutionalize fascism and moderate its excesses – he did close to nothing to mitigate those violent initiatives which may have become, at a certain point, inconvenient for his “institutional” profile, but that he couldn't regard other than as an organic and natural manifestation of social tension which could not be otherwise entirely absorbed or removed. In which case, it was better to be on the side administering violence.